The present invention relates to a continuous process for preparing racemic or optically active menthol, specifically L-menthol, in pure or enriched form by distillatively separating menthol from substance mixtures which comprise essentially menthol. This distillative separation is conducted in a dividing wall column.
Menthol, especially L-menthol, is one of the most important aroma chemicals globally and, owing to its cooling properties and to the fresh peppermint aroma, is used in a multitude of products.
The literature describes various purification processes for menthol. For instance, the person skilled in the art is aware, in addition to fractional distillation with and without steam, as described, for example, in DE 568 085 or DE 1 189 073 and U.S. Pat. No. 1,930,411, JP 27003884 or JP 32009869, of extraction processes and crystallization processes.
These processes are sometimes also used in combination, for example as combinations of crystallization and fractional distillation, or else in combination with chemical reactions or derivatizations.
GB 285,394 relates to a process for preparing racemic menthol by hydrogenating thymol, fractional distillation of the mixtures obtained therefrom and subsequent freezing of neomenthol out of the menthol fractions.
GB 285,833 describes a process for preparing thymol by fractional distillation of mixtures which have been obtained from the condensation of cresol with acetone, and, as well as thymol, comprise isomeric methylisopropylphenols.
U.S. Pat. No. 2,827,497 discloses a process in which diastereomer mixtures of menthol obtained by fractional distillation and fractional crystallization are subjected to an oxidation and then purified further by another fractional distillation.
EP 0 242 778 describes a process for separating diastereomer mixtures, including mixtures of menthol, isomenthol, neomenthol and neoisomenthol, by extractive distillation, i.e. by distillation with addition of more specific assistants, for example succinamide.
The processes described usually have the disadvantage that assistants are used (steam or extractive distillation), or solids are obtained. The fractional batchwise distillations are disadvantageous usually with regard to their yield of product of value, since the product is thermally stressed for a longer period.
EP 1 514 955 relates to a process for distillative workup of the electrolysis output of the electrochemical oxidation of 1,1,2,2-tetramethoxyethane with methanol to give trimethyl orthoformate in a liquid electrolyte, wherein a dividing wall column with 30 to 150 theoretical plates is used.
DE 103 30 934 discloses a process for continuously isolating citronellal or citronellol from a crude mixture comprising at least one of these compounds by rectification. Preference is given to using those starting mixtures which are obtained by partial hydrogenation of citral or citronellal.
DE 102 23974 relates to a process for continuously isolating two stereoisomeric isoprenoid alcohols, specifically nerol and geraniol, from a crude mixture by rectification, wherein the crude mixture is introduced laterally into a feed column, at least one draw column coupled to the feed column is provided, and a first and second isoprenoid alcohol are drawn off from the draw column. The feed and draw columns are coupled such that there is no cross-mixing of vapor and condensate at least in the region of the draw point of the isoprenoid alcohols.
The distillative purification of menthol, specifically of L-menthol to free it of its neoiso- and isomenthol diasteromers, is typically very complex especially owing to the very small boiling point difference of approx. 2° C. at ambient pressure.